In simplest terms, a "zero strain" stretch laminate web, as those terms are used herein, refers to a laminate web comprised of at least two plies of material which are secured to one another, either intermittently or substantially continuously, along at least a portion of their coextensive surfaces while in a substantially untensioned ("zero strain") condition. At least one of said plies is preferably in the form of a continuous web to facilitate continuous, high speed processing. The other of said plies may comprise a continuous web or discrete elements or patches secured to the continuous web at predetermined locations.
As used in the foregoing context, an "intermittently" bonded laminate web shall mean a laminate web wherein prior to the application of tension the plies are initially bonded to one another at discrete spaced apart points or one wherein the plies are substantially unbonded to one another in discrete spaced apart areas. Intermittently bonded laminate webs of the first type can be formed by passing two heat bondable plies through a heated patterned embossing roll nip or by applying discrete spaced apart areas of adhesive to one of the plies before bringing it in contact with the other ply, while an intermittently bonded web of the second type can be formed by feeding an adhesively coated apertured ply or scrim between a pair of substantially continuous plies. Conversely, a "substantially continuously" bonded laminate web shall mean a laminate web wherein prior to the application of tension the plies are initially bonded substantially continuously to one another throughout their areas of interface. Substantially continuously bonded laminate webs can be formed by extruding a first substantially continuous, thermoplastic adhesive ply directly onto a second ply while the first ply is in a heated condition, passing two heat bondable plies between a heated smooth surfaced roll nip or by applying a substantially continuous adhesive coating, spray or densely patterned melt blown to one of the plies prior to bringing it in contact with the other ply.
One of the plies employed in a "zero strain" stretch laminate web of the present invention is comprised of a material which is stretchable and elastomeric, i.e., it will return substantially to its untensioned dimensions after an applied tensile force has been released. The second ply secured to the elastomeric ply is elongatable, most preferably drawable, but is not necessarily elastomeric. Whatever its composition, the second ply will, upon stretching, be at least to a degree permanently elongated so that upon release of the applied tensile forces, it will not fully return to its original undistorted configuration. To the extent that the permanently elongated second ply is not secured to the elastomeric web after the stretching operation, the permanently elongated second ply expands in the z-direction between its points of securement to the elastomeric web when the elastomeric web to which it is secured returns to its substantially undistorted configuration in the x-y plane. The greater the distance between the adjacent points of securement in the x-y plane after stretching, the greater will be the degree of z-direction expansion in the resultant laminate web. Regardless of the degree of z-direction expansion, the resulting "zero strain" stretch laminate web is thereafter elastically extensible in the direction of initial stretching, at least up to the point of initial stretching.
While the term "zero strain", which is used herein to describe stretch laminate webs to which the present invention pertains, has not to Applicants' knowledge been used by prior art workers to describe webs of the aforementioned type, it will for consistency be hereinafter used throughout the present specification to describe such webs.
One very early execution of an intermittently bonded "zero strain" stretch laminate web is disclosed in U.S. Pat. No. 2,075,189 issued to Galligan et al. on Mar. 30, 1937. According to the disclosure of the aforementioned Galligan et al. patent, two superposed continuous plies of rubber, one of which is under tension and longitudinally stretched, are passed between a pair of pressure rolls traveling at the same peripheral speed. One of the rolls is provided with relatively small or narrow projections in a desired pattern, which projections cooperate with the second roll to press together into adhesive contact small portions of the two plies of rubber so that relatively closely spaced small areas of the superposed plies will be united in a pattern similar to that of the projections on the pressure roll.
According to Galligan et al., the roll cooperating with the roll having projections may be smooth, or instead it may be provided with mating projections similar to those on the other roll. The rolls are spaced apart, depending upon the combined thickness of the two plies of rubber, to a degree sufficient to provide the desired uniting pressure without undesirably thinning the rubber of the joined areas.
Upon issuance of the joined plies from the rolls, the tension on the stretched ply is relaxed, and as a result this ply contracts in length and also slightly expands in width. Since the unstretched ply intermittently bonded thereto cannot thus contract, it is drawn up from a longitudinal direction in puckers or crinkles 4. In the specific embodiment shown in FIGS. 1 and 2 of Galligan et al., the top or crinkled ply is designated by the numeral 1, while the stretched or backing ply is designated by the numeral 2. At 3 there appear narrow parallel joint lines at the points where the two plies have been united by the pressure.
In a succeeding step of the process disclosed in the Galligan et al. patent, the foregoing intermittently bonded composite comprising a two ply crinkled material is very highly stretched in a lateral direction (substantially parallel to the joint lines 3), the tension being sufficient to stretch the top crinkled ply 1 beyond its elastic limit. However, the applied tension remains within the elastic limit of the bottom or backing ply 2. If desired, the lateral stretching may be to a point as high as eight times the original width of the undistorted composite.
Since the top ply is laterally stretched beyond its elastic limit, its crinkles 4 are necessarily permanently thinned out in a lateral direction so that when the lateral tension on the laminate sheet is released, the superficial area of the material in any crinkle, when spread flat, will be much greater than that of the corresponding portion of the backing ply 2. As a result, when the backing ply 2 laterally contracts, the crinkles 4 on the top ply 1 are drawn up from a lateral direction, and since their superficial area is much greater than before, the contracting effect of the backing ply causes the crinkles to assume a highly irregular and distorted form between the joint lines 3, i.e., it produces z-direction bulking of the composite, as generally shown in FIGS. 5, 6 and 7. Galligan et al. suggest that the resultant "zero strain" stretch laminate material is particularly suitable for use in the making of bathing suits, bathing caps, shoes, aprons and other articles.
Another early execution of an intermittently bonded "zero strain" stretch laminate web, which is specifically suggested for uses such as toweling, wiping material and expendable garment material, is disclosed in U.S. Pat. No. 3,025,199 issued to Harwood on Mar. 13, 1962. In particular, Harwood suggests the formation of a scrim comprised of intersecting sets of threads or filaments 2 and 3 which are bonded to one another at their points of intersection to form a reticulated reinforcing network 1. A pair of nonwoven layers 4 and 5 of fibers are preferably attached to the opposite sides of the reinforcing network 1 formed by the intersecting threads.
The laminate web structure disclosed by Harwood is thereafter subjected to a stretching operation in one or more directions to permanently expand the nonwoven webs 4,5 secured to the opposed surfaces of the reinforcing network 1. According to Harwood, this may be carried out by stretching the laminate web crosswise (i.e., in the cross-machine direction) via suitable roll means or by appropriately guided conveyor chains equipped with means for gripping and applying opposed tensile forces to the side margins of the web (i.e., tentering apparatus). If lengthwise stretching of the laminate web is desired, Harwood teaches that this may be effected by cooperative low and high speed roll pairs.
Since the threads 2,3 used to form the reticulated reinforcing network 1 of Harwood are, in a particularly preferred embodiment, resilient, the network 1 tends to restore itself to a predetermined substantially undistorted configuration as soon as any tensile forces which have been applied to the laminate web are removed. As a result, the permanently expanded outermost plies 4 and 5 shown in the cross-section of FIG. 4 of the Harwood patent exhibit z-direction bulking in the unbonded areas 6 which coincide with the openings in the resilient network 1.
More recent executions of both intermittently bonded and substantially continuously bonded "zero strain" stretch laminate webs comprised of synthetic polymer plies and intended for single use or disposable apparel applications are disclosed in commonly assigned U.S. Pat. No. 4,107,364 issued to Sisson on Aug. 15, 1978 and commonly assigned U.S. Pat. No. 4,209,563 issued to Sisson on Jun. 24, 1980. The commonly assigned Sisson patents, which are hereby incorporated herein by reference, teach that the "zero strain" stretch laminate webs therein disclosed are particularly well suited for single use apparel applications because of their relatively low cost compared to conventional cloth materials. The Sisson patents further teach that such "zero strain" stretch laminates may be constructed in many different forms ranging from extremely lightweight versions suitable for lingerie applications to heavier versions suitable for apparel waistband applications.
In a preferred embodiment, Sisson's "zero strain" stretch laminate comprises at least one ply comprised substantially of synthetic polymeric filaments which are relatively elastomeric and at least one ply comprised substantially of synthetic polymeric filaments which are relatively elongatable but relatively nonelastic. In a particularly preferred embodiment the plies are bonded to one another to form a coherent laminate web.
As pointed out earlier herein, Sisson discloses two types of web bonding configurations: substantially continuous bonding, as can be accomplished via a heated smooth roll nip; and substantially intermittent bonding at a plurality of spaced apart points, as can be accomplished via a heated patterned embossing roll nip.
Laminate webs employing either bonding configuration are thereafter mechanically worked as by stretching, preferably substantially uniformly, in at least one direction followed by substantially complete relaxation to develop a low modulus of elasticity in the direction of stretching. In the case of the intermittently bonded laminate webs, the elongatable but relatively nonelastic ply is permanently elongated by the stretching operation. Accordingly, it is bulked and bunched between the intermittent bonds securing it to the relatively elastomeric ply when the applied tension is released, i.e., it is bulked to a significant degree in the z-direction to produce a "zero strain" stretch laminate web which is elastically extensible in the direction of initial stretching, at least up to the point of initial stretching. In the case of the substantially continuously bonded laminate webs, the permanently elongated polymeric filaments which are relatively inelastic do not retract when tension is released on the laminate web. Consequently they are caused to undergo looping, bulking and bunching on a much finer scale, i.e., between their bond points to the relatively elastomeric polymeric filaments when tension is released on the laminate web. While the z-direction bulking is less pronounced in such continuously bonded laminate webs, "zero strain" stretch laminate webs of the latter type are also elastically extensible in the direction of stretching, at least up to the point of initial stretching.
Numerous examples of "zero strain" stretch laminate webs employing either continuous or intermittent bonding configurations and methods for producing such webs are disclosed in the aforementioned commonly assigned Sisson patents.
Sisson's suggestion to employ "zero strain" stretch laminate materials in single use or disposable items of wearing apparel has been followed by a number of subsequent workers in the art. See, for example, U.S. Pat. No. 4,525,407 issued to Ness on Jun. 25, 1985, which discloses disposable diapers and surgical gowns incorporating one or more "zero strain" stretch laminate composites comprised of an untensioned elastic member intermittently bonded to an unstretched less extensible substrate, the resulting laminate being rendered elastic by stretching.
FIGS. 1-3 of Ness disclose a simple two layer "zero strain" stretch laminate web which is intended for use as an elastic bandage or wrap. The laminate web comprises a nonapertured elastic member 10 and an unstretched, nongathered substrate 12, which before it is stretched, is less easily extensible than the elastic member and which has less elastic recovery than the elastic member. The substrate and the elastic member are intermittently bonded at spaced apart points 14 in a regular or irregular pattern. The laminate web is thereafter stretched in the directions of the arrows shown in FIG. 2. Upon release of the applied tensile forces, the elastic member 10 causes puckering, i.e., z-direction bulking, of the permanently elongated substrate 12 between bonding points 14, as generally shown in FIG. 3. Like the aforementioned "zero strain" stretch laminate webs of Galligan et al., Harwood and Sisson, the resultant laminate web disclosed by Ness is thereafter elastically extensible in the direction of initial stretching, at least up to the point of initial stretching.
Another elastic composite web embodiment 30 is illustrated in FIGS. 5-8 of Ness. The latter embodiment employs a reticulated elastic element 20 having transverse strands 22 and longitudinal strands 24. The reticulated elastic element 20 of Ness appears to be generally similar to the resilient reticulated reinforcing member 1 disclosed in FIGS. 1-4 of the aforementioned Harwood patent. Like Harwood, Ness also employs a first substrate 28 having less extensibility than the elastic member 20 and less elastic recovery than the elastic member. A second substrate 30, which has substantially the same physical properties as substrate 28, and which "sandwiches" the elastic member 10, is also employed by Ness.
Substrates 28 and 30 of Ness are secured at least to the opposing surfaces of the reticulated elastic member 20 while the elastic member is in a substantially untensioned condition. The substrates 28 and 30 may, if desired, also be bonded to one another through the openings in the reticulated elastic member. According to the teachings of Ness, when the laminate web is thereafter stretched in the longitudinal direction, the substrates 28,30 undergo permanent elongation and may become delaminated from one another, but remain intermittently bonded to the reticulated elastic member 20 at the intermediate sites comprising the transverse and/or longitudinal strands of the reticulated member. Once tension on the web has been released, the reticulated elastic member 20 restores the web to the substantially undistorted configuration of the reticulated elastic member 20, thereby causing z-direction bulking of the permanently elongated substrates 28,30 between their spaced apart points of securement to the longitudinal strands 22 of the elastic member in a direction substantially perpendicular to the direction of stretching. The cross-section of the resultant elastic composite web of Ness shown in FIG. 9 is generally similar to that of the "zero strain" stretch laminate web shown in FIG. 4 of the aforementioned Harwood patent.
In addition to the foregoing "zero strain" stretch laminate web embodiments, FIGS. 9-12 of the Ness patent disclose the use of the elastic composite materials to provide extensible legband portions 136,137 and extensible waistband portions 138,139 along the opposed side edges and ends, respectively, of a disposable diaper. Such elastic composite materials may be incorporated into garments or bandages during manufacture and may, if desired, be stretched to provide subsequent elastic extensibility in the direction of initial stretching. According to Ness, the latter stretching operation may either be performed by the end user or applier of the product as it is being applied or it may be stretched during the manufacturing process.
An automated method for stretching a laminate web comprising a reticulated elastic 210 heat sealed to a pair of opposing plastic film layers 214,216 is disclosed in FIG. 14 of Ness. In the disclosed embodiment, the three layers comprising the composite are fed into a nip formed between a pair of smooth, heated, counter-rotating rolls 224,226 to heat seal the reticulated elastic to the two layers of film 214,216 to form a heat sealed three-layer composite 228. The heat sealed composite 228 is then fed into the nip formed between a second pair of counter-rotating rolls 230,232 which may be cooled to ensure that the thermal bonding is "set". The composite web 234 emerging from the second pair of counter-rotating rolls 230,232 is then fed into the nip of a third pair of counter-rotating rolls 236,238 rotating at a faster peripheral speed than the second pair of counter-rotating rolls 230,232 to effect drafting of the composite web 234 between the two pairs of rolls.
According to Ness, this drafting stretches the films 214,216 and ruptures the heat seal bonds which were previously formed between the films 214,216 through the apertures in the reticulated elastic scrim. Stretching the composite with elastic in the longitudinal direction may also, according to Ness, rupture the seal between the longitudinal strands and the film(s), leaving only the transverse strands bonded to the film layers 214,216. As the stretched composite 244 emerges from the third pair of counter-rotating rolls 236,238, the longitudinal or machine direction tension is relaxed and the composite 244 is fed to a windup 246 that is rotating at a peripheral speed approximately equal to the peripheral speed of the second pair of counter-rotating rolls 230 and 232.
While stretching a laminate web by applying tension to widely separated points of support, e.g., first roll pair 230,232 and second roll pair 236,238, does serve to permanently elongate the substantially inelastic film plies 214,216, Applicants have learned that the uniformity of elongation in such a "zero strain" stretch laminate web, as measured along the unsupported portion of the composite web 234, decreases as the distance between the first roll pair 230,232 and the second roll pair 236,238 increases. For any given distance between the first and second roll pairs, this nonuniformity becomes more pronounced as the difference in peripheral speed between the second roll pair 236,238 and the first roll pair 230,232 increases, i.e., as the composite web 234 undergoes a greater degree of stretching.
Applicants have further learned that these nonuniformity problems can be avoided or at least minimized by following one of the specific suggestions set forth in the aforementioned commonly assigned Sisson patents. Namely, to incrementally stretch the "zero strain" stretch laminate material by passing it through an incremental stretching system, such as the nip formed between a pair of meshing corrugated rolls which have an axis of rotation substantially perpendicular to the direction of web travel. The meshing corrugated rolls support the laminate web at plural closely spaced apart locations corresponding to the width of the corrugations during the stretching operation. This causes substantially uniform incremental stretching of each unsupported segment of the web between adjacent support points rather than highly localized stretching as often occurs when only the outermost extremities of the web are subjected to tension.
Sisson's suggestion to incrementally stretch a "zero strain" stretch laminate material by passing it between corrugated rolls to impart elastic extensibility thereto has also been followed by at least one subsequent worker in the art. See, for example, U.S. Pat. No. 4,834,741 issued to Sabee on May 30, 1989 and hereby incorporated herein by reference.
Sabee, like Ness, discloses a single use garment, such as a disposable diaper, employing a "zero strain" stretch laminate material comprising an untensioned elastomeric element secured between a pair of drawable elements in its opposed waistband and legband portions. The elastic elements 41 shown in FIG. 1 of Sabee are affixed in the waistband portions of the diaper web while in a substantially relaxed condition to a drawable topsheet web, a drawable backsheet web or both. The bonding configuration employed by Sabee may be either intermittent, as by passing the laminate material through a pressure nip formed between two rolls, one of which is heated and contains a plurality of raised points on its surface, or continuous, as by depositing a thin band of viscoelastic hot melt pressure sensitive adhesive onto one of the webs and thereafter pressing the hot melt pressure sensitive adhesive to the other web by passing the laminate through a pressure nip formed between a pair of smooth surfaced rolls.
Regardless of which bonding configuration is employed, the portions of the diaper web containing elastic web elements 41 are thereafter laterally stretched in the cross-machine direction by the meshing corrugations on pairs of corrugated rolls 31, as generally shown in Sabee's FIGS. 5 and 6. Simultaneously the coinciding portions of the drawable topsheet and backsheet webs in the area of elastic element attachment are incrementally stretched and drawn to impart a permanent elongation and molecular orientation thereto in the cross-machine direction. Because corrugated rolls 31 have their meshing corrugations aligned substantially parallel to the machine direction, incremental stretching of the web takes place in the cross-machine direction. Accordingly, the fully processed waistband portions of Sabee's diaper web are thereafter elastically extensible in the cross-machine direction, at least up to the point of initial stretching.
A similar machine direction stretching operation is preferably carried out with respect to the opposed legbands, which include untensioned elastic elements 42, by passing the diaper web of Sabee between another pair of meshing corrugated rolls 89, as generally shown in FIGS. 12 and 13. Because corrugated rolls 89 have their meshing corrugations aligned substantially parallel to the cross-machine direction, incremental stretching of the web takes place in the machine direction. Accordingly, the legband portions of Sabee's diaper web are thereafter elastically extensible in the machine direction, at least to the point of initial stretching.
While Applicants have confirmed that Sisson's suggestion to use corrugated rolls to incrementally stretch a "zero strain" stretch laminate web works reasonably well when large portions or the entire surface of the web are subjected to treatment, unexpected problems have been encountered when only predetermined isolated portions of such a web are to be subjected to treatment or when only predetermined portions of the web comprise a "zero strain" stretch laminate composite. For example, when only the waistband and legband portions of the diaper web shown in FIGS. 5, 6, 12 and 13 of the aforementioned Sabee patent are comprised of "zero strain" stretch laminate material, there is a significant tendency for the "zero strain" portions of the diaper web to slip or contract in a direction parallel to the desired direction of stretching as they pass between the meshing corrugated rolls.
Slippage or contraction of the "zero strain" portions of the laminate web in a direction parallel to the desired direction of stretching renders the incremental stretching process less effective. In such case, the relatively nonelastic ply or plies to which the elastic element is bonded do not undergo as great a degree of elongation as would otherwise be the case if the laminate web were not allowed to undergo overall contraction in a direction parallel to the direction of web stretching. As a result, the activated or stretched portions of the resulting "zero strain" stretch laminate web exhibit a lesser degree of shirring and elastic extensibility when the finished articles are cut from the web and applied to or worn by the user.
The aforementioned problem becomes more and more serious as the size of the discrete elastic elements contained within the web or the portions of the stretch laminate web to be incrementally stretched get smaller and smaller.